It is often of significant importance to be able to detect very small surface defects in critical componentry. Flaws of 0.010" or less may grow resulting in catastrophic or life threatening failures in aircraft components or other materials, objects or members when such members or components are subjected to high stress. Known ultrasonic and electromagnetic nondestructive test techniques have not proved effective in detecting defects of 0.010" in length reliably and repeatably.
Typical eddy current sensor employs an eddy current coil to sense the presence of surface flaws or defects. The resolution of such sensors depends upon the size of the coil, the electrical properties of the material under test, the frequency of the coil energizing signal in addition to other factors. These sensors depend upon a change in the impedance of a coil for defect detection. Detection capabilities of eddy current coil sensors are therefore limited due to the operational characteristic that coil sensors integrate over the coil area and reliable detection of flaws below 0.010" length by 0.005" depth has been problematic. (See K. J. Kryzwosz, R. E. Beissmer, and J. E. Doherty; 14th Symposium on NDE; San Antonio, Tex.; Apr. 19-21, 1983).
A recently disclosed eddy current sensor for surface defect detection utilizes a loop of wire as a driver and a receiver coil normal to the plane of the driver coil. The receiver coil is operative to detect tangential fields resulting from eddy current flow along edges of a defect. (See R. Langman, British Journal of NDT. pp. 8487, March 1979) The disclosed technique has several disadvantages; notably, the use of a circular coil receiver limits the resolution of the detector and a circular defect which is concentric with the coil may not be detected.
To scan a large area with a small coil eddy current probe necessitates time consuming mechanical scanning techniques which are costly to implement and which may fail to precisely locate small defects.